OBJECTIVES: 1. Continued characterization of RadLV/VL3 structural proteins purified with the aid of monoclonal hybridoma antibodies and studied by SDS-PAGE, by crossreactivity with antibodies to the corresponding proteins of other murine type C viruses, and by tryptic digestion and high-pressure liquid chromotography of the hydrolysate peptides. 2. Characterization of the temperature-sensitive mutants of RadLV/VL3 recently isolated by Dr. Jean Emeny, with emphasis on their thymotropism and leukemogenecity in vivo, complementation patterns for replication at nonpermissive temperatures in vitro, and attempts to identify whether a structural protein or the viral reverse transcriptase is the temperature-labile gene product. 3. Further evidence for the viral etiology of radiogenic lymphomas, as outlined previously. 4. Studies of the pathogenesis of virus-induced and of radiation-induced thymic lymphomas, as outlined previously. 5. Studies of the relevance of virus-impaired immunocompetence in leukemogenesis. 6. Studies of the evolution of autonomy in autochthonous lymphomas.